Caesar IV online was run by the publisher Sierra. Sierra no longer exists as a independent publisher and the server Caesar IV online was on is gone. Sierra was absorbed into Activisition-Blizzard after they merged a few months ago and most of their old games like Caesar IV are no longer supported.

The chances of a new Caesar IV online server being setup are essentially nil.

No, the online portion of the game stored all the results on a remote server at Sierra (originally at TM but later moved). Sierra is gone, the servers are gone.

A infinite number could play, because you weren't actually playing live online against someone else. You were playing the game like you usually did and periodically information about your city was sent over the internet to the remote server where it was added to your useraccount name and the results were then available for others to see. The server would keep track of who was doing the best and rank the players.